Ok. Back to logic.
Gotta have water.
Its gotta flow.
Gotta have air to cool it down.
Im always leery of the thermostat. I say remove the thermostat, drive with out it. Fill with water after starting but before driving because it will afford you immediate flow through the engine so it insures the water side of the equation also. Rev engine (while observing inside radiator) will insure the water pump is actually doing its thing too. Dont get any on ya.
If it overheats then it doesnt have (water) or airflow. If it doesnt? Well then.
I know.....we're chasin after new parts now.
Without starting this elimination process over again it seems like the only thing you didnt do is flush the block out. Everything else is new.
Running without a thermostat can be a problem though. If the coolant runs through the radiator too quickly, it won't cool down. The thermostat slows down the coolant flow so it can lose heat through the radiator.